The present invention relates to vapor permeation curable coating compositions and more particularly to a curing chamber with constant gas flow environment which is designed especially to cure said coating compositions.
Vapor permeation curable coatings are a class of coatings formulated from aromatic-hydroxyl functional polymers and multi-isocyanate cross-linking agents wherein an applied film thereof is cured by exposure to a vaporous tertiary amine catalyst. In order to contain and handle the vaporous tertiary amine catalyst economically and safely, curing chambers have been developed. Generally, such curing chambers are substantially empty, rectangular boxes through which a conveyor bearing the coated substrate passes. Provision is made for entrance and exit of vaporous tertiary amine, normally borne by an inert gas carrier such as nitrogen or carbon dioxide, for example, and means are provided at the inlet and outlet of the chamber to enhance containment of the vaporous tertiary amine catalyst within the chamber. The inlet and outlet containment means further restrict the entrance of oxygen into the chamber because oxygen can create an explosive condition with the vaporous tertiary amine catalyst. Cure of such coatings is so rapid that no external source of heat is required.
Representative examples of past curing chambers are set forth in U.S. Pat. Nos. 3,851,402, 3,931,684, and 4,294,021. Of particular note in the patented curing chambers is the provision made at the inlet and outlet for containment of the vaporous tertiary amine curing gas within the chamber. For example, U.S. Pat. Nos. 3,851,402 and 3,931,684 provide moist air curtains at the inlet and outlet which moist air curtains along with a source of suction are designed to minimize escape of tertiary amine gas from within the chamber. Somewhat different is the design in U.S. Pat. No. 4,294,021 which calls for the exhaust fan to create a slight negative pressure to induce gas flow within the chamber in the direction of the exhaust duct which is located near the exit of the chamber. It is noted by the patentees that air is dragged by the conveyor from the inlet and such flow of air along with the vaporous amine circulates from the entrance of the chamber to the exhaust duct where the gas is withdrawn for recirculation. The patentees further note that the negative pressure created at the exhaust duct near the outlet also creates a flow of air from the exhaust end of the chamber into the chamber itself. No provision in this patent is seen for minimizing air flow into the chamber and, to the contrary, the design appears to encourage the flow of air into the chamber.
While prior curing chambers certainly have performed adequately in the marketplace, many problems exist with prior designs. One problem with prior designs is the loss of amine vapor. Another problem is the inability to prevent air from entering into the curing portion of the chamber. A further disadvantage is the inability to operate at rapid conveyor belt speeds. The present invention addresses these and other deficiencies in the prior art and provides a unique chamber as will be more fully appreciated by the description contained below.